1. Field of the Invention
The present invention relates to footwear. The invention concerns, more particularly, athletic footwear having an upper and a sole structure with a bifurcated construction.
2. Description of Background Art
Conventional articles of athletic footwear include two primary elements, an upper and a sole structure. The upper is usually formed of leather, synthetic materials, or a combination thereof and comfortably secures the footwear to the foot, while providing ventilation and protection from the elements. The sole structure often incorporates multiple layers that are conventionally referred to as an insole, a midsole, and an outsole. The insole is a thin, cushioning member located within the upper and adjacent the sole of the foot to enhance footwear comfort. The midsole, which is traditionally attached to the upper along the entire length of the upper, forms the middle layer of the sole structure and serves a variety of purposes that include controlling potentially harmful foot motions, such as over pronation; shielding the foot from excessive ground reaction forces, and beneficially utilizing such ground reaction forces for more efficient toe-off. In order to achieve these purposes, the midsole may have a variety of configurations, as discussed in greater detail below. The outsole forms the ground-contacting element of footwear and is usually fashioned from a durable, wear resistant material that includes texturing to improve traction.
The primary element of a conventional midsole is a resilient, polymer foam material, such as polyurethane or ethylvinylacetate, that extends throughout the length of the footwear. The properties of the foam midsole are primarily dependent upon factors that include the dimensional configuration of the midsole, the material selected for the polymer foam, and the density of the midsole material. By varying these factors throughout the midsole, the relative stiffness, degree of ground reaction force attenuation, and energy absorption properties may be altered to meet the specific demands of the activity for which the footwear is intended to be used.
In addition to foam materials, conventional midsoles may include, for example, stability devices that resist over-pronation and moderators that distribute ground reaction forces. The use of foam midsole materials in athletic footwear, while providing protection against ground reaction forces, may introduce instability that contributes to a tendency for over-pronation. Pronation is the inward roll of the foot while in contact with the ground. Although pronation is normal, it may be a potential source of foot and leg injury, particularly if it is excessive. Stability devices are often incorporated into foam midsoles to control the degree of pronation in the foot. Examples of stability devices are found in U.S. Pat. No. 4,255,877 to Bowerman; U.S. Pat. No. 4,287,675 to Norton et al.; U.S. Pat. No. 4,288,929 to Norton et al.; U.S. Pat. No. 4,354,318 to Frederick et al.; U.S. Pat. No. 4,364,188 to Turner et al.; U.S. Pat. No. 4,364,189 to Bates; and U.S. Pat. No. 5,247,742 to Kilgore et al. In addition to stability devices, conventional midsoles may include fluid-filled bladders, as disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 to Marion F. Rudy.
As an alternative to the conventional midsole structures discussed above, various articles of footwear include a spring within the sole structure. For example, U.S. Pat. No. 4,566,206 to Weber discloses an article of footwear having a spring positioned in the heel area of the sole structure. The spring includes an upper portion and an intermediate portion that are joined to a lower portion to form acute angles with the lower portion. U.S. Pat. No. 5,367,790 to Garnow et al. discloses an article of footwear with a spring having a collapsible longitudinal arch. The spring includes an upper plate and a lower plate joined together approximately two-thirds of the distance from the rear of the footwear to the front of the footwear. A similar configuration is disclosed in U.S. Pat. Nos. 5,701,686 and 6,029,374 to Herr et al., which also discloses a forefoot plate that provides a spring in the fore portions of the footwear. U.S. Pat. No. 4,492,046 to Kosova discloses an article of footwear with a sole structure that incorporates a spring wire.